Acid zinc electroplating and electrolyte therefor



Patented Dec. 4, 1951 ACID ZINC ELECTROPLATING AND IELECT-ROLYTE THEREFOR Allan E. Chester, Highland Park, Ill., assignor to Peer .& Company, (Chicago, 111., a corporation of Delaware No' Drawing. Application June19,'1948,

' Serial No. 34,127

' 12 Claims.

This invention relates to an acid zinc electrolyte and to v a method of electrodepositing 1 zinc from acid zinc plating baths.

This application is a continuation-in-partof my application Ser. No. 441,694 filed May 4, 1942,

.;which matured into U. S. Patentg2,4.43,599,.dune

.One of the objects of the present. invention is to provide a newand improved method of electrodepositing zincifrom an acidzinc. electroplating bath.

Another object is to .provide a, method ofpro- 'ducing bright zinc. depositsfrom an acid zinc plating bath.

"Still another object vis'toprovide a method of producing image bright'zinc deposits from an' acid zinc plating bath at ordinary temperatures .(e. g., 65 degrees F. to 95"degrees"F'.

A further object of the invention isvto provide a newand improved method of producingzinc detposits from an acid platingbath athi'gh current densities without. burning.

An additional object of the, invention is. to provide a new and improved methodof producing brilliant zinc deposits directly out of an acid zinc 3 platingv bath.

,Another object is to provide a simplet pe of .}acid zinc electrolyte characterized by excellent operating efiiciency at higher current densities -\.without burning the platedarticle.

. A further object is to providea new and improved electrolyte forproducing ,imagebright lzinc deposits directly from anacid zinc; plating ,bath. Other objects will appear hereinafter.

These objects areaccomplished in accordance lwithl-this invention by. incorporating with an acid szinc electroplating bath certain organicaddition -agents. Accordingto one embodiment of the invention excellent results are obtained :in the electrodeposition of the zinc toproduce image. sbright zincplates directly from an acid bathwithrout-the necessity for any subsequent chemical .treatment. By another embodimentof the 1 in- .-vention;gray zinc-deposits areobtained with bet- ..ter operatinggefficiency outof an acid-bath and with less burning. Still another embodiment. of

.g-the invention relates to theprovision ofv simpler ..forms l of improved electrolytes adapted for ithe .purposes of the invention.

.The primary organic addition-agentwhich is, .lemployedlfor the purpose of theinventionis an taldonic, acid, preferablyegl-uconic acid .or-ia gluconate that is soluble in the acid zinc bath. The

moreor: less .water insoluble; aldonates,-.;e. g., :lcal- E;ci m-:; 1uconate,ta ,so improv 1 the ap a in acha acteristics of the acid zinc bath. The addition .agentmay be added in the form otan aqueous solution of rthealdonic acid or asia salt thereof, the cation of which does not adversely affect the bath. The .aldonicacid addition agent can'lie added in theform of azinc aldonate previously prepared by the reaction of zinc oxide with an aldonicacid. Thealdonicoacid can be obtained bythe oxidation of an aldose, or in any other *suitablamanner. .If gluconic acidisused as such it is preferably employed in the form of .an aqueous solutionhaving a concentration of about of 'gluconicacid. Other. examples ofsacids andsalts thereof whichmaybe used aregalactonic, arabonic,.xy1onic.and mannonic. The aldonic acid, when used in sufiicient amountihas a pronounced antiburn effect and produceslimproved plating .efficiency.

Asecond addition agent, which.is essentialjfor the production of bright plates in the practiceof the invention, is an alkylated naphthalenesulionic acid which may be added in the form ofthe' free sulfonicacid ora solublesalt thereof. [The preferred addition agent of this classis .isopropyl naphthalene sulfonic acid sodium salt which is available in. a commercial formrof 95% purity.

Neither thealdonic acid alonenor the alkylated naphthalene :sulfonic. acidalone will produce a bright zinc plateioutof the acid bath. When thesetwo addition agents are employed together, however, .a specific and surprising. result is ob- .tained.

Excellent results have been obtained atroo m temperatures degrees F.), and equally good :results have been obtainedat temperaturesoififi degrees F. .to degrees F. At temperatures :above degrees Fhmaintenance of a low lead content (below 5 milligrams of dissolved lead per liter) and agitation are important factors inob- .taining-a bright plate directly-out of the bath, yas describedand claimed in the copending application of Chester, Main and HansenfSen. No. 34,129, filed of even .date herewith. mercial' operations. the bath becomes heated nor- In vcom-- mally by the current whichispassing through it, especially when high current. densities, are used.

vItis'not ordinarily desirable to operateat tem-- peratures ab0vedegrees F. because v oflthe operating hazards to the. operators. .JThis'. isespecially true. where the object being plated: is a continuous strip which might-break andhaveto be removed fromthe bath.' The samedifficulty arises inrcase individual articles. are being-plated,

vas in piece plating, .and'one or. more ofthese-drops from its rack into the bath. .vvill .be apparent zinc per liter.

if the bath were heated to a very high temperature it might be necessary to cool it before the strip could be repaired or the article could be removed from the bath, as the case might be.

Very desirable plates can be obtained in accordance with the invention over a wide pH range of 1 to 5.5. The optimum pH range for bright plates is 2.5 to 4.0.

The invention can be practiced by adding one or more of the addition agents to any of the ordinary acid zinc electrolytes which may or may not contain other addition agents. Such zinc electrolytes and theirmethod of preparation are generally well known.

Glucose in combination with the alxylated naphthalene sulfonic acid added in accordance with this invention will not produce bright zinc plates. Glucose or other aldose, therefore, is in no sense the equivalentof gluconic or another aldonic acid for the purpose of the present invention.

In the past it has been customary to prepare acid zinc electrolytes containing salts other than zinc salts, bulfers, glucose or other reducing sugars and various types of organic colloids which have been alleged to have a beneficial effect in the bath. For the purpose of the present invention, however, it has been found that excellent results can be obtained by making the necessary additions to a Simple electrolyte consisting of zinc sulfate and water. The zinc content of the bath for the purpose of this invention ma vary rather widely, but in continuous plating it is preferably within the range of 50 to 150 grams of In piece plating where speed of operation is not necessarily an important factor it is possible to use a bath having less than 50 grams per liter of zinc.

' foot are employed.

The invention may be employed over a wide range of current densities. In continuous plating, in accordance with the invention, it is preferable to operate at current densities within the range of 100 to 250 amperes per square foot. Hull cell plates have been made, however, with electrolytes prepared in accordance with the invention showing the antiburn effects of gluconic acid alone and image bright surfaces with gluconic acid and an alkylated naphthalene sulfonic acid over a current density range from 2 to 60 amperes per square foot. There appears to be no particular limit on the current densities which may be employed, except as they are limited by the equipment used and the particular article plated. Because of the fact that high current densities may be used in the practice of the process, correspondingly high rates of zinc deposition may be obtained with resultant high production schedules.

Zinc plates may be obtained in accordance with the invention in a varying range of thickness of deposit. The preferred range of thickness in continuous plating of sheets or strips is within the range of 0.000025 to 0.0005. On the other hand, in the plating of wire it may be desirable to deposit much greater amounts of zinc, and this has been done in the practice of the invention. Thus, plates on wire were obtained in thicknesses from 0.002 inch to 0.004 inch (1.2

ounces to 2.5 ounces per square foot). It will be understood that where ductility is a factor there may be limits of thickness beyond which a deposit will tend to chip or crack. Such limits can readily be determined in any particular case by routine experiments. At the lower operating temperatures, i. e., below degrees F2, and with both types of the aforesaid addition agents bright deposits are obtained without agitation. In most continuous plating processes sufilcient agitation is inherently present: If the amount of agitation is sufficient so that the desired brilliance is obtained without streaking, the rate of agitation can be increased while still maintaining the brilliance of the zinc deposit.

The invention will be further illustrated by the following examples in which the quantities are stated in parts by weight unless otherwise indicated.

Example I An electrolyte was prepared by mixing the following ingredients:

450 grams zinc sulfate.

30 grams sodium acetate.

4 cc. of a 25% solution in water of Aerosol OS (a product of American Cyanamid Company containing 95% isopropyl naphthalene sulfonic acid sodium salt and 5% inerts).

32 cc. of 50% gluconic acid in water.

Water sufficient to make a liter of solution.

The bath was purified by electrolyzing it as described in Transactions of Electrochemical Society, vol. 80 (1941) pages 391 and 392. The DH of this electrolyte was maintained within the range of 2.5 to 4.0.

This electrolyte produced image bright deposits when electrolyzed at normal current densities common to the art, such as 25 to 300 amperes per square foot.

Image bright deposits were obtained out of this bath at ordinary temperatures of 75 degrees F. without agitation.

Example II To a zinc sulfate bath containing 1920 grams of zinc sulfate and 8000 cc. of Water there was then added varying amounts of zinc gluconate, these amounts being respectively 40 grams, grams, 200 grams, 480 grams and 800 grams. By a series of experiments it was determined that the optimum pH was around 2.5 to 3.5. Hull cells were made from these baths at a current density of 1 ampere per square foot for 10 minutes at '74 degrees F. The Hull cell plates showed that the addition of the zinc gluconate had a pronounced antiburn effect above 15 grams per liter especially in the highest current density range of the Hull cell tests, 1. e., above about 50 amperes per square foot.

To the previously described purified zinc sulfate bath containing 1920 grams of zinc sulfate dissolved in water to produce eight liters of solution there was then added 5, 15, 25, 40, 65 and 80 cc. per gallon of a 25% by weight solution of isopropyl naphthalene sulfonic acid sodium salt. Hull cell plates were made with each of these baths in a manner similar to that previously described and again it was observed that image brightness was not produced. An eifect on the grain structure of the deposit was noted above 15 cc. per gallon, apparently due to a decrease in crystal size.

Two acid zinc plating baths were then prepared by (1) adding 1920 grams of zinc sulfate ate-saw and 320 grams of zinc gluconate to 8000 cc. of water, and .(2) adding 480 grams of zinc gluco nate to the same quantities of zinc sulfate and water, and adjusting the pH to 2.47. These baths were electrolyzed 17 hours at 1 ampere per square foot for purification. To' these baths there was then addedvarying amounts of isopropryl naphthalene sulfonic acid sodium salt, namely, 5 cc. per gallon, cc. gallon, cc. per gallon, cc. per gallon, cc. per gallon and 86 co per gallon of a 25% by weight solution of 95% is'opropyl naphthalene sulfonic acid sodium salt. The Hull cell plates made from these baths at '74 degrees F. all showed image brightness in some areas and hence indicated that this image brightness was due to the combined effect of the gluconate and the isopropyl naphthalene sulfonic acid sodium salt. All of these Hull cell plates showed brightness in the lowest current density range. The optimum efiect over the widest range of current densities was obtained with 25 cc. per gallon of the i'sopropyl naphthalene sodium sulfonate solution in the second bath which contained 60 grams per liter of the zinc gluconate.

Example III An acid zinc electroplating bath was prepared by-dissolving zinc sulfate in water in proportions suchthat the quantity of zinc in soluble form. was within the range of 50 to 150 grams per liter, then adding to this electrolyte cc. per gallon of a composition prepared by mixing together:

(a) 8D cc. of 20% Aerosol OS (made by dissolving 125 grams of Aerosol OS in 500 cc. of water) and 3.8 grams ofcadmium sulfate made up to a gallon with water; and

(b) 20 cc. of 50% gluconic acid and 1 cc. of ammonium bisulfite made up to a gallon with Water. I

This bath produced bright zinc plates at tempcratures of 75 degrees F. to 95 degrees F.

- Example IV "In the bath described in Examples I and III lactic acid in chemically equivalent proportions was used instead of the gluconic acid or gluconate.' [Good results were obtained with this bath in piece plating at a temperature of 90 degrees F. and 120 'amperes per square foot. This bath was not so sensitive to impurities as the gluconic acid bath but it did not produce a bright plate directly out of the bath even with the Aerosol OS present. A bright plate was produced,

however, by dipping this plated product into a second bath consisting of of nitric acid in water. l

The current used in the various tests described in the examples was direct current. It will be understood that the invention can be employed with other types of current and abnormal wave forms, including pulsating direct current such as may be derived by superimposing alternating current on direct current or by the use of various types of rectifier arrangements. For example, a

ratio of 5 volts A. C. to 8.4 volts D. C. has been used satisfactorily. Where a superimposed current is employed the optimum results can ordinarily be obtained with less agitation.

Although the invention has been specifically described with respect to isopropyl naphthalene sulfonic acid sodium salt, it will be understood that other water soluble alkylated naphthalene sulfonic acids and salts thereof can be employed in the practice of the invention, including the but'yl, isobutyl, amyl andisoamyl derivatives and the ammonium, potassium and other water sold ble alkali metal and alkaline earth metal salts of the various alkylated naphthalene sulfonicacids. Inorganic salts such as are sometimes present in the commercially available alkylat'ednaphthalene sulfonic acid salts have no adverse effect on the results. Thus, the isopropyl naph thalene sulfonic acid sodium salt is commer cially available in a form which is pure and contains 5% inorganic salts. The optimum results have been obtained with about 1.5 to 2.5 grams of the alkylated naphthalene sulfonic acid derivative per liter of electrolyte. These alkylated naphthalene sulfonic acid derivatives also lower the surface tension of the resultant electr'olyte, and this can be used as a measure of the quantity of the alkylated naphthalene sulfonic.

ner previously described. The thickness of the.

coating, the nature of the surface of the article plated and'the type of article are all factors to be considered. Thus, a very thin coat of zinc deposited on a dull surface will not have the brightness of a thicker coat.

The gluconic acid employed in the practice of the invention existsin several forms and the invention contemplates the use of one or more of these forms, or mixtures thereof, including mixtures, of the lactone forms. Commercial gluconic acid is available as a 50% aqueous solution of approximately 99% gluconic acid and 1% glucose. The presence of the glucose is undesirable because it tends to build up in the bath to cause gumminess or stickiness. conate described in the examples is substantially free of glucose. This may be prepared by heating a commercial 50% by weight gluconic acid solution to degrees F. sufliciently long to melt any crystalline lactone that might be present, then adding one mole of lead-free zinc oxide for every two moles of gluconic acid, heating to V degrees F. with agitation and holding for 10 minutes until solution is complete. The resultant product is cooled to 120 degrees F. in the liquid phase and poured into large stoneware vessels and cooled until a waxy solid forms containing some supernatant liquor. The liquor which is mainly glucose and water is removed by filtration and the residue is placed in drying trays. At this point it may be washed with ice water to dissolve traces of residual glucose. The product is then dried by heating at temperatures below 250 degrees F. until dry and hard, after which it is ground in a hammermill or other suitable means. The resultant product is stable and non-sticky. The quantities of this material em- .ployed with optimum results has been within the Thus, in a particular test the surface The zinc glu-.,

for example, chromic acid and various combinations of chromic acid with other chemicals such as are sometimes used to brighten zinc plates made by the alkaline cyanide process.

The invention produces unusual results in the plating of zinc from acid baths. By the practice of the invention plates have been produced which have been comparable to bright chromium plates. .So far as is known, these results have not heretofore been attained in this art. Additionally, the invention provides improved performance of acid plating baths, excellent zinc deposits, operation at high current densities and other advantages which will be apparent to those skilled in the art.

Throughout the specification and claims where the addition agents are referred to broadly as a gluconate and an alkylated naphthalene sulfonate" it will be understood that these expressions comprehend the addition of the free gluconic acid and free alkylated naphthalene sulfonic acid as well as soluble salts thereof to the acid zinc electroplating bath.

The reaction products of zinc oxide and aldonic acids are disclosed and claimed as compositions of matter in my co-pending application, Serial No. 180,500, filed August 19, 1950.

The invention is hereby claimed as follows:

1. An acid zinc plating bath comprising zinc sulfate dissolved in an acidic aqueous liquid containing a member from the group consisting of aldonic acids and aldonates in a quantity at least effective to produce antiburn effects in said bath.

2. An acid zinc plating bath comprising zinc sulfate dissolved in an acidic aqueous liquid containing zinc gluconate and an alkylated naphthalene sulfonate dissolved in said bath.

' 3. An acid zinc plating bath comprising 50 to 150 grams per liter of zinc sulfate dissolved in an acidic aqueous liquid containing a gluconate and an alkylated naphthalene sulfonate dissolved in said bath in quantities effective to produce a bright plate directly out of the bath.

4. An acid zinc plating bath comprising zinc sulfate dissolved in an acidic aqueous liquid containing a gluconate and an alkylated naphthalene sulfonate dissolved in said bath, said bath having a pH between 1.0 and 5.5.

5.;An acid zinc plating bath comprising zin sulfate dissolved in an acidic aqueous liquid con taining a gluconate and an alkylated naphthalene sulfonate dissolved in said bath, said bath having a pH between 2.5 and 4.0.

6. An acid zinc plating bath comprising essentially zinc sulfate dissolved in an acidic aqueous liquid containing zinc gluconate and isopropyl naphthalene sodium salt dissolved in said bath.

plate when the latter is dipped in a nitric acid solution.

9. A method of zinc plating which comprises electrodepositing zinc from an acid bath containing zinc sulfate and a member from the group consisting of aldonic acids and aldonates in a quantity at least effective to inhibit burning of the zinc plate.

10. A method as claimed in claim 9 in which the electrodeposition is carried out with agitation at a temperature below degrees F.

11. A method as claimed in claim 9 in which the electrodeposition is carried out at current densities in excess of 30 amperes per square foot.

12. A method of producing a bright zinc plate which comprises electrodepositing zinc from an acid zinc sulfate plating bath containing lactic acid and an alkylated naphthalene sulfonate in a quantity effective to produce a bright plate when the plated article is subjected to a secondary treatment with an oxidizing acid, and thereafter, subjectin said plated article to said secondary treatment.

ALLAN E. CHESTER.-

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 570,554 Jordis Nov. 3, 1896 694,658 Meurant Mar; 4, 1902 967,200 Kern Aug. 16, 1910 1,818,229 LutZ Aug. 11, 1931 2,227,508 Nelson Jan. 7, 1941 2,245,086 Bray et a1 June 10, 1941 2,451,426 Bair et al Oct. 12, 1948 2,452,003 Weber Oct. 19, 1948 2,485,563 Chester et al Oct. 25, 1949 OTHER REFERENCES Metallische Uberzuge, by Willi Ma'cher, 2nd Ed., Copyright vested in Alien Property Custodian 1944, published by J. W. Edwards 1945, page 235. 

1. AN ACID ZINC PLATING BATH COMPRISING ZINC SULFATE DISSOLVED IN AN ACIDIC AQUEOUS LIQUID CONTAINING A MEMBER FROM THE GROUP CONSISTING OF ALDONIC ACIDS AND ALDONATES IN A QUANTITY AT LEAST EFFECTIVE TO PRODUCE ANTIBURN EFFECTS IN SAID BATH. 